A UK flu expert said the work could be useful in directing research, even if it was not directly transferable.

The World Health Organization (WHO) says there are three to five million cases of flu worldwide each year, and it is linked to 250,000 to 500,000 deaths annually.

Every year a vaccine is produced to act against the most commonly circulating types of flu.

But the researchers say vaccines are "variably effective", largely because of the way viruses change.

'Unambiguous results'

In this study, they looked at the effect of a protein called granulocyte macrophage-colony stimulating factor (GM-CSF).

It is known to boost the activity of alveolar macrophages (AM), the first line of defence against inhaled organisms and molecules.

They compared mice given GM-CSF with some who were not.

The animals were infected with lethal doses of a flu virus.

Transferring it into humans can be quite difficultProfessor John Oxford,, Virologist, Queen Mary, University of London

All the untreated mice lost weight and died within days. But all those treated survived, and while they lost weight initially, they regained it after a short period.

The effect was seen for a number of different flu strains, including swine flu (H1N1).

Dr Homayoun Shams, from the University of Texas, who led the study, said: "Such unique and unambiguous results demonstrate the great potential of GM-CSF and may be the remedy for a critical public health priority - developing strategies to reduce the morbidity and mortality from influenza.

He added: "Despite the widespread use of vaccines, influenza causes significant morbidity and mortality throughout the world, and those with poor immune systems are particularly more susceptible—such as very young, elderly or immunocompromised individuals.

"Unlike a vaccine, GM-SCF does not rely heavily on the body's ability to mount an immune counter-attack against a specific antigen or virus strain, but enhances the speed of local responses to virus infection and delicately balances the host immune responses."

GM-SCF is already used to treat people with neutropenia, a blood disorder.

Professor John Oxford, a virologist based at Queen Mary, University of London, said it was "cutting edge" research.

But he added: "Transferring it into humans can be quite difficult. We don't know the sort of problems that could occur if you start meddling with the immune system, as we saw with the Northwick Park trial [in which six men became seriously ill after taking a drug which was meant to "retune" the immune system].

"So first we'll see how it pans out in mice, then see how it can possibly apply to humans.